Sarcopenia defines the loss of muscle mass, strength and function occurring during aging [1]. Muscle mass loss starts from the age of 30 years at a rate of 3-8% per decade and accelerates from 60 years of age. This loss reaches up to 35-40% in elderly over 70, and hence, sarcopenia is especially prominent in elderly.
Muscle mass preservation can only be achieved through an adequate stimulation of protein synthesis and/or inhibition of proteolysis. Several factors contribute to muscle protein synthesis among which the most important is the availability of amino acids (serving as building blocks for the newly synthesized proteins) and the activation signal generated by anabolic amino acids or anabolic amino acids derivative (e.g. citrulline, leucine, essential amino acids (EAA), and creatine).
However, protein intake is decreased in elderly, which leads to inadequate postprandial availability of amino acids. Furthermore, aging is characterized by a decreased muscle sensitivity to the anabolic effect of essential amino acids, especially leucine [2, 3]. This lesser responsiveness to leucine is associated with a decrease in the activation of intracellular pathways controlling muscle protein synthesis [4, 5]. Hence, to counteract the deleterious effect of aging on muscle protein synthesis, it is necessary to simultaneously target the bioavailability of amino acids, especially essential amino acids, by increasing the intake of high-quality protein (rich in essential amino acids, especially leucine), and the muscle response to anabolic amino acids by improving muscle sensitivity to these amino acids.
Of interest, aging is also characterized by a high prevalence of vitamin D deficiency (serum vitamin D levels below 25-50 nmol/L) and insufficiency (serum vitamin D levels below 75 nmol/L) [6, 7]. Low serum 25-(OH)D concentrations are associated with an increased risk of sarcopenia in elderly [8].
In the future, vitamin D deficiency could become a major health problem: only recently it was stated [9] that between 40 and 45 percent of the German population could be vitamin D insufficient, with an additional 15 to 30 percent deficient, thereby putting them at risk at a variety of health problems. In addition, current recommendations of daily intake of vitamin D are of 5 micrograms for adults under 50 years old, and 10 micrograms for adults older than 50 years and are not adequate to ensure vitamin D sufficiency (serum vitamin D levels equal or above 75 nmol/L) in the general population. Hence, there is a need to significantly increase these recommendations (at least to double them) and preferably to raise them to a daily intake of 25 micrograms or more depending on gender, age, activity level, and other factors.
Vitamin D deficiency in adults is reported to precipitate or exacerbate osteopenia, osteoporosis, muscle weakness, sarcopenia [11], loss of physical function, fractures, common cancers, autoimmune diseases, infectious diseases and cardiovascular diseases. There is also some evidence that vitamin D may reduce the incidence of several types of cancer and type-1 diabetes.
The science supporting the action of vitamin D on muscle strength, as well as the vitamin's role in immune health, is sufficiently robust to have merited a positive opinion from the European Food Safety Authority's (EFSA) Panel on Dietetic Products, Nutrition and Allergies (NDA) [10].